This article describes the US agency system and gives an account of the Supreme Court’s jurisprudence on delegating rule-making authority to agencies. Building on the case law, the article attempts to draw a picture of the legitimacy of agencies and their relations to the three branches of government in the US constitutional system. The US agency model serves as a baseline for the second part of the article, where a description is given of the evolution of agencies in the EU and the questions arising from the delegation of powers to agencies. The aim of the article is to highlight the common features of the two system, keeping in mind that any kind of comparison between the US and the EU agency system is necessarily flawed due to the fundamental differences between the constitutional settings. The EU agencies exist in a supranational-intergovernmental system that is a sui generis form of governance, thus a comparison with a federal or a national state setting is never without challenges.